The present invention relates to a sealable, opaque multilayer film which is composed of a base or support layer comprising a propylene polymer and a sealable surface layer having a low minimum sealing temperature.
Films of this kind are used as packaging films in high-speed packaging machines, for example, in horizontal forming, filling and sealing machines. In the packaging process, the film which enters into the machine as a flat web is bent about its longitudinal axis so as to wrap the product to be packaged and is shaped into a tubular casing. The sealable surface layer forms the inside of the tubing. The edge zones of the film are placed one on top of the other in such a way that inside is in contact with inside. By the application of heat, a strip-shaped sealed seam (fin seal) is then produced, which projects from the tubular package and extends parallel to the longitudinal axis of the tubing. For portioning the product which has been filled in and forming individual packages, sealed seams are produced which extend, in mutually spaced arrangement, transversely to the longitudinal axis of the tubing and the tubular casing is severed in the region of these transverse seams transversely to the longitudinal axis of the casing.
It has been found that customary polypropylene-based films are hardly suitable for this application. Especially in modern, high-speed forming, filling and sealing machines, in which the film is conveyed in the horizontal direction at a speed exceeding 40 m/min, machine runability and slip properties of the film must be particularly good.
The sealable, opaque polyolefinic multilayer film according to published European Patent Application No. 0 114 311, on which the present invention is based, is formed of a base layer comprising a propylene polymer and at least one sealable layer comprising a polyolefin resin, a low molecular-weight resin which is compatible with the polyolefin resin, a propylene homopolymer and a polydiorganosiloxane. It is also intended for use as a packaging film in horizontal forming, filling and sealing machines and actually shows good processing characteristics in high-speed packaging machines.
It has been found, however, that the machine runability and slip properties of this film are only good when it is used in the unprinted state. It is frequently required to apply a printing to the packaging film. In these cases, the film must be subjected to a corona discharge treatment to render it printable. By this treatment, the surface tension of the film is increased and adhesion between film surface and printing ink is thus improved. This pretreatment has, however, the great disadvantage that this film loses its good running characteristics in high-speed packaging machines.